1. Field of the Invention
This invention relates to an optical interferometer of the point-diffractive type, and more particularly, to a point-diffraction interferometer incorporating separate signal and reference beam paths.
2. Prior Art
The point-diffraction interferometer (PDI) was first described by Raymond N. Smartt and J. Strong in the Journal of the Optical Society of America, vol. 62, 1972, page 737. The theoretical basis for operation of the PDI is discussed in several articles, notably by Raymond N. Smartt et al. in the Japanese Journal of Applied Physics, Vol. 14, Suppl. 14-1, pp. 351-356, 1975; and by C. Koliopoulos et al. in Optics Letters, Vol. 3, pp. 118-120, September 1978. The usefulness of the PDI for testing large optical systems in-situ is discussed in a paper by Raymond N. Smartt in Interferometry, G. W. Hopkins ed., Proceedings of the SPIE, vol. 192, p. 35, 1979. A PDI of the prior art is presently marketed by Ealing Beck Ltd. of Watford, England, and is available in the U.S. through the Ealing Corporation, of South Natick, Mass.
FIG. 1 is a schematic representation of the prior art PDI. As seen in FIG. 1, the PDI is an elegant and simple interferometer well suited for embodiment in a small, rugged package. FIG. 1 shows that the PDI is placed in a converging beam 1 of light produced by the optical system which is to be tested. It is not necessary that the incident beam be temporally coherent light from a laser source, since the PDI is a common path interferometer. However, a laser source is ideal for use with a PDI.
The aberrated signal beam 2 is brought to a focus near a small (typically 5 microns diameter) pinhole aperture 3 located in a semi-transparent optical thin film deposited on substrate 4. Some of the optical energy contained in the incident beam passes through, and is diffracted by, pinhole aperture 3. Pinhole aperture 3 creates an expanding, diffraction-limited reference beam 5, consisting of spherical expanding waves 6. The remainder of the incident beam passes unchanged, but with reduced itensity, through coated substrate 4 to form diverging beam 7, which retains the aberrational content 8 of the original signal beam 2. Interference between the diffracted reference wavefront 6 and the signal wavefront 8 yields interference fringes.
Interference fringes produced by the PDI are interpreted in the same manner as interference fringes produced by a Twyman-Green interferometer. Aberration of the signal beam is directly indicated by the shape of the interference fringe contours. Focus error is indicated by fringes which have a circular contour. This arises if the signal beam converges to focus either ahead of, or beyond, the PDI pinhole. If focus error is removed, the remaining wavefront errors associated with the signal beam will be revealed in the form of fringes which deviate from straight lines. The nature of the deviation from straight lines will indicate the presence of optical aberrations (spherical, coma, astigmatism, etc.,) and/or manufacturing defects in the optics being tested.
In the PDI, fringe spacing is governed by the distance between the pinhole aperture and the centroid of focused energy in the point spread function of the incident beam. Fringe orientation is governed by the relative orientation of the focal position of the incident beam with respect to the pinhole aperture. If the separation between the focal point of the incident beam and the pinhole aperture is large, closely spaced fringes will result; if the separation is small, the fringes will be widely separated. It is easy to adjust fringe spacing with the PDI by a lateral translation of either the PDI pinhole aperture or the focal position of the incident beam. However, fringe visibility will vary because the amplitude of the signal base point spread function is not constant with radius.
If a converging signal beam is diffraction-limited, or nearly so, the point spread function will consist of an Airy disc surrounded by faint rings. In this case, little optical energy will be available for creation of a reference wavefront, except with the Airy disc. It will thus be necessary to position the PDI pinhole aperture within the Airy disc in order to generate a reference wavefront which has sufficient amplitude to produce fringes with acceptable visibility. However, the resulting fringes will be widely spaced. For a typical diffraction-limited signal beam, only 2 or 3 straight fringes of acceptable contrast may be generated. Any attempt to increase the number of fringes by increasing the separation between the focal spot and the pinhole will result in a loss of fringe contrast. This effect is independent of the focal ratio of the signal beam or the physical size of the Airy disc.
In the PDI, balance between the intensity of the signal and reference beams is achieved by optically attenuating the intensity of the signal beam. There will only be one radial distance from the center of the point spread function where the intensity of the reference beam produced by the PDI pinhole aperture will be ideal for best fringe contrast. This radial distance, in turn, will govern the number of fringes which are observed.
In summary, fringe contrast and fringe spacing are not independently variable in the prior art PDI. It is not possible to generate high contrast, closely spaced fringes for incident signal beams with low wavefront errors. This situation presents a problem for automated fringe analysis computer programs, which typically require 8 or more fringes in order to generate a meaningful contour map of the optical surface or wavefront under investigation. It is therefore an object of this invention to enhance the operational characteristics of the prior art PDI so as to provide for independent adjustment of fringe spacing, orientation, contrast, and intensity regardless of the quality of the signal beam.
Independent adjustment of fringe spacing, orientation, contrast, and intensity may be facilitated by splitting the incident signal beam into separate signal and reference beams, as in a Mach-Zehnder interferometer. FIG. 2 is a schematic illustration of a Mach-Zehnder interferometer of the prior art. In FIG. 2, a high quality collimated incident beam 10 produced by laser 9 is split into signal beam 12 and reference beam 13 by beamsplitter 11. The two beams are directed toward beam combiner 17 by folding mirrors 14 and 15. An optical component of unknown quality 16 is spaced in a signal beam 12 just prior to beam combiner 17, thereby modifying the wavefront of signal beam 12. Interference between the modified signal beam and the reference beam is observed in combined beam 18. A laser source must be used in a Mach-Zehnder interferometer, due to the difference in optical path lengths along the signal and reference beam paths caused by the presence of the test component.
The existence of separate signal and reference beam paths is a major advantage. Each beam in a Mach-Zehnder interferometer may be independently adjusted for intensity and direction, thereby permitting adjustment of fringe spacing, orientation, contrast, and intensity. An object of this invention is therefore to incorporate separate signal and reference beam paths into a PDI, thereby providing for fringe adjustability in a manner similar to a Mach-Zehnder interferometer.
The reference beam in a Mach-Zehnder interferometer is generated externally by large, expensive, diffraction-limited collimation optics. By contrast, in a PDI an inexpensive pinhole aperture serves to create a reference beam by spatially filtering the incoming signal beam. The advantages gained in a PDI through reductions in size, complexity, and cost are retained in the split-path PDI.
The signal beam in a Mach-Zehnder interferometer is generated internally when the reference beam encounters the unknown optical component. For a PDI, the signal beam is generated externally. Constraints on the physical size of the optical component or system being tested vanish when these can be located externally. Thus, an important advantage is gained for a PDI by eliminating the need to place the test component within the signal arm of the interferometer. This advantage is retained for the split-path PDI. The only requirement is that the component or system being tested produce a converging beam. Since many optical systems ranging from small lenses to large telescopes are designed to produce converging beams, this is not much of a restriction. When this is not the case, conversion optics of suitable quality may often be used.
In summary, it is the object of this invention to incorporate split beam paths into an interferometer of the point-diffractive type, thereby providing the flexibility to vary fringe spacing, orientation, contrast, and intensity. The split beam paths are highly advantageous and are not known in the prior art PDI. It is the further object of this invention to retain the best features of the prior art PDI, such as: (1) a low cost pinhole aperture for generating a reference wavefront; (2) the ability to perform non-invasive or in-situ testing of many kinds of optical components and systems; and (3) small physical size consistent with providing for a compact, rugged design suitable for rough handling on a daily basis. In keeping with the intention of providing a compact, rugged design, and to provide greater flexibility to the user, it is a further object of this invention to incorporate a compact, solid state laser diode light source and a double-pass beamsplitter such that double-pass interferometry may be performed, while keeping the overall package size small.